Duolingo (/ˌdjuːoʊˈlɪŋɡoʊ/DEW-oh-LING-goh) is a free language-learning platform that includes a language-learning website and app, as well as a digital language proficiency assessment exam. Duolingo offers all its language courses free of charge. As of November 2016, the language-learning website and app offer 66 different language courses across 23 languages; with 22 additional courses in development. The app is available on iOS, Android and Windows 8 and 10 platforms with about 120 million registered users across the world.[2][4][5]

Inspiration for Duolingo came from two places. Luis Von Ahn wanted to create another program that served two purposes in one, what he calls a "twofer".[8] Duolingo does this by teaching its users a foreign language while having them translate simple phrases in documents.

Forbes' staff member Parmy Olson revealed the other incentive towards creating Duolingo. Von Ahn was born in Guatemala and saw how expensive it was for people in his community to learn English. Severin Hacker (born in Zug, Switzerland), co-founder of Duolingo, and Von Ahn believe that “free education will really change the world”[9] and wanted to supply the people an outlet to do so.

Duolingo started its private beta on November 30, 2011, and accumulated a waiting list of more than 300,000 users.[14] On June 19, 2012, Duolingo launched for the general public. Due to popular interest, Duolingo has received many investments including a $20 million Series C round of investment led by Kleiner Caufield & Byers and a $45 million Series D round of investment led by Google Capital.[15] Duolingo has 60 staff members, of whom many were Google employees,[16] and operates from an office in the Pittsburgh neighborhood of Shadyside near Carnegie Mellon's campus.[17][18]

On November 13, 2012, Duolingo released their iOS app through the iTunes App Store.[19] The application is a free download and is compatible with most iPhone, iPod and iPad devices.[20] On May 29, 2013, Duolingo released their Android app, which was downloaded about a million times in the first three weeks and quickly became the #1 education app in the Google Play store.[21] Duolingo then released both a Google Glass App (glassware) and support for Android Wear.[22]

Core services of the platform are available for free. Duolingo uses only very limited advertising in its Android app. There are no subscription fees for the tutorials. However, there were instances when the platform has been used for paid translation purposes.[23] In July 2014, Duolingo started a language certification service, Test Center, as a new business model. In June 2015, a Duolingo spokesperson confirmed that the company has been backing away from the translation business and in the future will instead focus on language certification and other (not yet announced) business opportunities.[24] According to Luis von Ahn, in order to offset the over $40,000 a day cost of developer salaries, server costs, etc., Duolingo now charges a fee to maintain a streak freeze, at the request of users.[25]

To earn money, Duolingo originally employed a crowd sourced business model, where members of the public were invited to translate content and vote on translations. The content came from organizations that pay Duolingo to translate it. Documents could be added to Duolingo for translation with an upload account which had to be applied for.[26] On October 14, 2013, Duolingo announced it had entered into agreements with CNN and BuzzFeed to translate articles for the companies' international sites.[27]

Duolingo provides written lessons and dictation, with speaking practice for more advanced users. It has a gamified skill tree that users can progress through[19] and a vocabulary section where learned words can be practiced.

Users gain "experience points" (XP) as they learn a language, such as when they complete a lesson.[33] Skills are considered "learned" when users complete all the lessons associated with the skill. Users win one point for each correct answer, and lose one for each error, and validate the lesson when they reach 10 points. In an earlier version, users would start with four "lives" on early lessons[33] and three on later lessons, a "life" being lost with each mistake.

Duolingo also includes a timed practice feature, where users are given 30 seconds and twenty questions and awarded a skill point and seven or ten additional seconds (time depends on the length of the question) for each correct answer.[34] This timed practice feature is only available after purchase in the Lingot store for ten lingots, the currency the site uses. As the goal of Duolingo is to get people to learn the language, each skill (containing between 1 and 10 lessons) has a "strength bar" that corresponds to the computer's estimate of how strongly certain words or constructions still exist in the user's memory. After a certain duration of time, strength bars fade, indicating a need for a user to refresh/re-study that lesson, or to "strengthen weak skills." Courses can teach upwards of 2,000 words.[35] Duolingo uses a data-driven approach to lesson planning.[36] At each step along the way, the system measures which questions the users struggle with and what sorts of mistakes they make.

For each language, Duolingo also has a "Progress Quiz" feature. This is an extended quiz that you can purchase in the Lingot store for 25 lingots. It measures your language learning progress, with 5 being the perfect score. This quiz is not adjusted to your level and it only tests your overall language competence.

The efficacy of Duolingo's Spanish course has been reviewed by a study commissioned by the company. Conducted by professors at City University of New York and the University of South Carolina, the study estimated that 34 hours on Duolingo may yield reading and writing ability of a US first-year beginners' course university semester, which takes in the order of over 130 hours. The research did not measure speaking ability. It found that a majority of students dropped out after less than 2 hours of study.[37] The same study found that Rosetta Stone users took between 55 and 60 hours to learn a similar amount.[38]

A lingot (a portmanteau of the words lingo and ingot) is the currency of Duolingo.[39] Users gain lingots by leveling up, finishing a skill, maintaining a usage-streak for a total of seven days, inviting others to Duo via social media, and by uploading documents to the Immersion. Items available in the lingot store include power-ups, practice and bonus skills. Users of the mobile apps can also customize the appearance of the Duolingo mascot, which is by default the green owl.[39] This feature is not available on the browser version. A hidden bonus skill included in some courses is "Christmas", which is only available in the period surrounding Christmas. Other Bonus Skills include "Flirting" and "Idioms and Proverbs".

Instead of slowly adding additional languages, CEO Luis von Ahn announced on May 29, 2013, that they would create the tools necessary for the community to build new language courses, with the hope to introduce more languages and "empower other experts and people passionate about a specific language to lead the way".[40] The result was The Language Incubator,[41] which was released on October 9, 2013.[42][43] In addition to helping the community create courses for widely spoken languages, the Duolingo Incubator also aims to help preserve some of the less popular languages such as Latin, Mayan and Basque.[44] The first course entirely created by the Duolingo community through the Incubator was learning English from Russian, which launched in beta on December 19, 2013.[45] Other courses created by the Duolingo community include English from Turkish, Dutch and Hungarian, as well as French and Brazilian Portuguese from Spanish.

The Incubator has three phases. First, when sufficient interest to contribute to a new course has been received from volunteers fluent in both languages (a requirement for application), the course begins in "Phase 1: Not Yet Released". The second phase, "Phase 2: Released in Beta," begins when the course has been fully prepared and is ready for open beta testing. Finally, "Phase 3: Graduated from Beta" is when a course is considered relatively stable. The reason complete courses remain in the Incubator is that moderators/contributors can continue to improve the course. For example, if a student gets a question wrong but notices there was an error on the program's part, which either misled the student or counted a correct answer wrong, they may submit a report detailing what happened.

Duolingo launched Duolingo Test Center on July 22, 2014. It is an online language certification platform that can be taken on the Web, iOS or Android, where tests are proctored through microphone and camera. There are two types of tests: Quick Test and Duolingo Certificate. Quick Test let you try some questions from the Duolingo Certificate. Duolingo Certificate requires 20 minutes to finish, costs 49 US$, and is graded within 48 hours on a scale between 0 and 10.[52] Duolingo Certificate allows just Google Chrome browser. According to a study conducted by University of Pittsburgh, there is a substantial correlation between the scores on Duolingo Test Center and TOEFL iBT.[53][54]

Upwork has adapted the Test Center score as one of their official qualifications with which freelance workers can prove their English fluency.[55][56] Duolingo is also collaborating with LinkedIn to allow the score to be easily incorporated into a user's résumé page.[52] Duolingo is working with twelve US universities, including Carnegie Mellon University, to study if the score can be reliably used as one of their admission qualifications.[57][58]

Duolingo has been used in schools. For example, in Costa Rica and Guatemala, Duolingo has been used in public schools as a pilot project run by the government.[59]

In January 2015, Duolingo released Duolingo for Schools, to provide teachers with a centralized dashboard that can display their students’ progress.[32] The dashboard helps teachers understand each individual student's weaknesses and strengths at each skill, and helps them optimize their language-teaching methods.[60]

Tinycards is a flashcards application made by Duolingo, wherein people can make and share flashcard sets.[61] After select users were invited to a closed beta test, Duolingo released the app for iOS devices on July 19, 2016. Users can sign in with their Duolingo account and immediately choose which sets they are interested in. Some include "US Capitals" or "Human Skeleton Body." Additionally, sets from a user's language course taken on Duolingo will be available for flashcard review. Currently progress on Tinycards does not synchronize with progress on Duolingo.

In 2013, Apple chose Duolingo as its iPhone App of the Year, the first time this honor had been awarded to an educational application.[64] Also, Duolingo won Best Education Startup at the 2014 Crunchies,[17] and was the most downloaded app in the Education category in Google Play in 2013 and 2014.[65] In 2015, Duolingo was announced the 2015 award winner in Play & Learning category by Design to Improve Life.[66]

^Users of the EU cannot be involved in the translation of any documents: "Terms and Conditions of Service". Duolingo. Temporary Restrictions on Users from the European Union. Users within the European Union are not presently allowed to submit materials for translation or translated materials to Duolingo. While these users can continue to use the educational services offered through the Website, they will not be involved in the translation of any documents. If you submit a request for translation or translated materials to Duolingo, you thereby warrant and represent that you are not currently within the European Union, did not translate the document within the European Union, and will not be within the European Union when your translation request has been finalized.